A conventional virtual-machine monitor (VM monitor) typically runs on a computer and presents to other software the abstraction of one or more virtual machines. Each virtual machine may function as a self-contained platform, running its own “guest operating system” (i.e., an operating system hosted by the VM monitor). The guest operating system expects to operate as if it were running on a dedicated computer rather than a virtual machine. That is, the guest operating system expects to control various computer operations and have an unlimited access to the computer's physical memory and memory-mapped I/O devices during these operations. For instance, the guest operating system expects to maintain control over address-translation operations and have the ability to allocate physical memory, provide protection from and between guest applications, use a variety of paging techniques, etc. However, in a virtual-machine environment, the VM monitor should be able to have ultimate control over the computer's resources to provide protection from and between virtual machines.
Thus, an address-translation mechanism is needed that will support attempts of a guest operating system to control address translation while enabling a VM monitor to retain ultimate control over address translation and computer resources.